Discretion comes down to four things: how big the toy is, what it looks like, how much noise it makes and how easy it is to store and take with you. A small vibrator rattling around in a toiletry bag isn't discreet, and neither is a quiet vibrator the size of a hairdryer.
The most discreet forms are lipstick vibrators, small bullet vibrators and finger vibrators. If you want something worn under your clothes, a small panty vibrator or a vibrating egg is the alternative, and if you want something with no motor at all, there are Kegel balls. Whatever the form, a travel lock – which disables the buttons – is the single most important feature for stopping the toy from starting by itself.
Look at what the manufacturer actually publishes. The word “quiet” without a decibel figure says little, and “discreet” isn't a specification. Dimensions, weight, travel lock and charging connector, on the other hand, are details you can compare.